As is well known, polarisation mode dispersion is a phenomenon of distortion of the optical signal that propagates in single mode optical fibres, i.e. optical fibres that carry signals in a fundamental propagation mode constituted by a pair of degenerate modes with orthogonal polarisation.
This phenomenon is, as is well known, linked to the dual degeneration of the fundamental mode of the optical fibres. Construction imperfections in real fibres (core ellipticity, external stresses, etc.) entail different group velocities for the two degenerate modes of the optical signal transmitted at one end of the fibre and hence distortions of the signal received by a receiving device (receiver) located at a second end of the fibre.
Therefore, it is important to develop accurate techniques for measuring PMD to characterize, for example, telecommunication systems that use optical fibres to carry signals, in particular in the case of high speed or high capacity transmission systems, for instance at bit rates up to 10 Gbit/sec or higher.
The PMD of the fibres is measured, for instance, by connecting a segment of optical fibre (fibre) 11 to be characterized to appropriate measuring instruments or devices 10 (FIG. 1).
The paper “Polarization Mode Dispersion of Short and Long Single-Mode Fibers” by N. Gisin et al, published on the “Journal of Lightwave Tech.” Vol.9 on Jul. 7, 1991 discloses, for instance, an instrument 10 for measuring the PMD, which comprises an optical source 21 (FIG. 1, FIG. 2), able to generate optical signals to be applied to the fibre 11, a Michelson interferometer (interferometer) 25 connected to the second end of the fibre 11, able to apply Differential Group Delays or DGD to the received optical signal and, at the output of the interferometer 25, a measuring device 27 able to convert the optical signals into electrical signals and to process, on the basis of program modules stored therein, the electrical signals received.
According to the prior art, the electrical signals obtained downstream of the interferometer 25 are graphically represented by the measuring device 27 by means of an interferogram having in its x-axis time offset values introduced by the interferometer and in its y-axis values of intensity of the photo-current generated.
In particular, according to the mentioned paper, said interferogram is to be interpreted as a Gaussian distribution of PMD of the fibre, so that the PMD itself can be determined according to the standard deviation of the Gaussian curve that best interpolates the interferogram.
This type of interpretation of the electrical signals derives from the fact that to the fibre 11 is applied a propagation model called “waveplate model”, which consists of considering the real optical fibre as a great number of birefringent plates positioned in cascade with respect to each other and having their main axes oriented randomly.
This model is used, as is well known, to assess signal time-of-flight, in the hypothesis that at each interface between a waveplate and the next one the optical signal is coupled randomly on the main axis of the downstream waveplate.
Based on the known model and in the realistic hypothesis that the length of the fibre is far greater than the coupling length between the polarizations, the prior art concludes that the measured graphic representation corresponds to a Gaussian curve with a standard deviation equal to twice the standard deviation of the optical signal due to PMD and hence indicative of the PMD of the fibre.
However, it is believed that the measurements obtained taking the known model as a reference do not precisely correspond to real PMD values.
In essence, it is believed that the conclusions of the prior art, which are based on the identification of the distribution of the times-of-flight of the optical signal with the distribution of the electrical field are wrong.
It is also believed that the PMD measurements obtained according to the known model are consequently only approximately correct.